Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of a rotor shaft drives a generator rotor either directly (“directly driven”) or through the use of a gearbox. The gearbox (if present), the generator and other systems are usually mounted in a nacelle on top of a wind turbine tower.
When wind turbines are erected, the tower is generally constructed first and placed upon a foundation. Then the nacelle, typically containing the generator and the gearbox (if present), is usually mounted on top of the tower. Then, the hub may be hoisted and mounted. Finally, the blades are hoisted one by one from a ground level towards the rotor hub and mounted therein. In an alternative method, one or more blades may be pre-assembled with the hub when lifting the hub. Depending on the wind turbine configuration, the combination of nacelle and hub may be hoisted to the top of the tower, after which the blades are mounted to the hub.
The blades may be installed in a variety of ways, e.g. in a substantially vertical manner or substantially horizontally or at other inclined angles. Particularly, in offshore wind turbines, mounting a blade from above the hub may be very complicated, or at least require an very large, high capacity crane and jack-up barge. Such an installation method may thus be very expensive and depending on the depth of the sea may simply be impossible.
It is known to hoist a complete rotor assembly, i.e. the hub with blades, and mount it to e.g. the nacelle. But in order to mount a complete rotor assembly, a large surface area is required, which is typically not available in the case of offshore wind turbines.
It is further known to hoist and mount an incomplete rotor assembly on the nacelle, e.g. the hub with two blades and subsequently, hoisting and mounting the remaining blade. In these cases, the rotor with the two blades is normally mounted with the two blades pointing upwards, i.e. “bunny ears” configuration. There is thus no need for rotating the wind turbine rotor as the third blade could be vertically mounted from below.
However, in order to be able to perform these rather complicated and time consuming installation steps, the prevailing wind speed has to be below a predetermined value. The period of time depends on the expected length of the installation operation and a safety factor to be taken into account.
During certain seasons, windows of several hours in which the wind does not reach the predetermined maximum wind speed may not be available very often. In practice, this may mean that personnel and maintenance equipment, including e.g. expensive cranes and jack-up barges, may be in stand-by during days or even weeks. This may represent an enormous cost.
It is known to mount each of the plurality of blades substantially horizontally or substantially vertically. This means that individual installation steps may require less time and may be performed at higher winds, thus increasing the time windows available for installation. However, such methods require rotation of the hub between the mounting of one blade and another. In order to correctly position the hub, torque is required for rotating the wind turbine rotor after mounting one blade in order to mount the next one. When not all blades have been mounted onto the hub, the hub is not balanced, so that the weight of one or more blades has to be carried upwards when rotating the hub.
Application of a very high torque may therefore be necessary for rotating the hub.
The torque may be delivered by using the gearbox (when present) with an external power supply for rotating the rotor. Such a system may be used in combination with a locking pin for maintaining the rotor in a desired position for installation. This is not possible in case of directly driven wind turbines as no gearbox is provided between rotor and generator. Even though the generator may be suitable for being operated as a motor, it will generally not be able to deliver the required torque to rotate the hub when the hub is unbalanced, i.e. when at least one but not all the blades have been mounted.
It is an object of the present disclosure to provide systems and methods that at least partially reduce one or more of the aforementioned drawbacks, leading to improved methods of mounting a hub on top of a tower especially suitable for direct drive offshore wind turbines.